Triosephosphate isomerase, an isomerase catalyzing an uphill reaction, offers a prime opportunity to stabilize and probe key kinetic and structural features of a Michaelis complex. This proposal capitalizes on our recent advances in stabilizing the Michaelis complex, and our optimization of a range of biophysical probes for the system, so that individual kinetic events and intermediate(s) can be studied. X-ray diffraction at 1.2Angstroms of the Michaelis complex shows a highly compressed active site with unusually short hydrogen bonds and an unexpected substrate torsional state. Recent data, utilizing three spectroscopic methods, indicate that loop opening and product release is rate determining for enzymatic throughput, and invite the hypothesis that loop opening may be triggered by the completion of the enzymatic reaction. The following issues are now ripe for pursuit. (1) Mutants in which the general base glutamic acid 165 and the general acid histidine 95 are replaced with alternative hydrogen bond partners are catalytically inactive. How does selection of the active site amino acids influence compression in the Michaelis complex and polarization of the substrate? (2) What are the chemical species in the Michaelis complex, and how do their concentrations depend upon temperature? Does the proposed enediolate chemical intermediate have a significant population, as might be expected based upon isotope washout? The substrate, while "pinned" through hydrogen bonds to the active site, appears to be mobile at the carbon centers, suggestive of an active rearrangement. Are the chemical reaction rates faster than the loop's motion, as suggested by the minimal primary isotope effects? (3) What are the conformational and ionization changes in the active site of the protein for the Michaelis complex, and what changes accompany the progress along the chemical reaction coordinate? How is the general base E165 positioned for the initial stage of the reaction? (4) What implications would changes in the ligand or mutations in the strongly conserved loop residues have for loop opening rates and for catalysis? These questions would be pursued through solid state NMR, vibrational spectroscopy and X-ray diffraction experiments, both static and dynamic .